According to recently released data from NPD, the iPhone 4S was the most popular US smartphone during the third quarter of 2011. But the 4S’s success is not simply limited to the States. According to data compiled Kantar Worldpanel ComTech, the most popular smartphone in the UK during the month of October was the iPhone 4S, an incredible feat given that it didn’t even go on sale there until mid-October.

All told, Apple’s share of the UK smartphone market in October checked in at 42.8%, besting Android which comes in at 35%. This is the opposite dynamic of the market in the US where Android devices cumulatively outsell Apple iPhones, despite the fact that the 4S is the most popular individual device in the country.

Apple’s sales were mostly from inside its own camp, however, Kantar Worldpanel ComTech points out. Amazingly, 75 percent of those who purchased a new 4S in October had previously owned an iPhone device, and 14 percent were upgrading from the iPhone 4, which means that a good percentage of them were probably buying out contracts early to get the device.

While some might interpret this as a sign that Apple has trouble attracting new customers, we view it as a positive indication that once a customer joins the iPhone fray, the user experience is such that he/she becomes a lifelong iPhone user. On the flipside, there doesn’t appear to be any allegiance amongst Android owners to any particular handset or manufacturer. A HTC EVO owner today may very well be a Samsung Galaxy S II owner tomorrow. And while that’s certainly great for consumers who can hop around looking for the next big thing, it doesn’t bode well for Android manufacturers whose devices are increasingly becoming easily replaceable commodities.